These words, attributed to Rutherford, provide a focus for this article. To what extent can one ‘see’ that most important of all the cations – the hydrated proton? What do different eyes – different experimental techniques – see and how do the views from these differing points of observation differ or complement one another? Our search will encompass the three states of matter – solid, gas and liquid solutions. And our ‘eyes’ will include X-ray and neutron diffraction, nuclear magnetic resonance, pulsed-electron-beam high-pressure mass spectrometery, infrared and Raman spectroscopy and relaxation techniques.

In addition we must consider the ‘mind's eye’: quantum simulations and theoretical inferences. But first we will place our search in historical perspective.

Historical Perspective [2–5]

The recognition that the class of compounds generally recognized as acids had, in common, the element hydrogen can be attributed to Davy and Liebig. In 1838 Liebig defined acids as ‘compounds containing hydrogen, in which hydrogen can be replaced by a metal’ [2]. This concept unified a body of empirical knowledge that had been accumulating: the sour taste; the solvent power; the ability to turn blue vegetable dyes red; the ability to combine with bases (or alkalis) to form salts and water; the existence of acidic compounds which did not contain oxygen, e.g. HF, HC1, HBr etc. (a discovery by Davy (circa 1810) that laid to rest the oxygen theory of acids proposed by Lavoisier).